Yes, indeed, OTK boots are extremely fashionable right now. I suspect at some level they owe their popularity to the hint of raffishness they offer - a little more daring than ordinary knee boots but without the dreaded Vivian effect of the thigh boot. A naughty-but-nice median, especially when in softer materials like suede and in greys or earth tones. Wearable anywhere.
Why any boots have open toes is a mystery to me - and not one I am the least interested in solving. I'd prefer they just vanished from view. No sense and not very appealing - a double whammy.
I have a couple of pair of OTK boots - one in dark brown suede, one in dark grey suede, both pull-on styles with flattish heels. I wear them a lot, and not just for their ease in getting them on, although that is a big plus in my book. I have plans to get yet another pair, in soft black calf. Very nice looking and useful boots, these over-the-knee styles..